I take it you are referring to this :
I didn't say the US had locked up 160,000 political dissidents. Learn to read before you accuse people of pulling facts out of their ass.
I will try to spell it out for you.
China has approx 5x as many citizens as the US (low-end estimate), and the post claimed that China had 800,000 "political prisoners" (no sources given so we have to take the word of an anonymous poster). For the US to achieve the same percentage of population locked up as "political prisoners" it would only have to lock up 800,000/5 = 160000.
According to the CIA World Factbook (ho ho) :
China has ~1,298,847,624 people, therefore 800,000 political prisoners amounts fo 0.062% of the population (if my math is correct, may well not be, I am hurrying).
The US has ~293,027,571 people, therefore 160,000 political prisoners would amount to 0.055% of the population. I underestimated the US population side on my original figures. 180,000 people would be need to be arrested for their political beliefs for the US to be as bad as the (unverifiable) 800,000 that China is alleged to have detained. It's not that many if you can accept that over a thousand people were (illegally) detained for their political beliefs during Bush's visit to NYC.